Some of you might be familiar with the S&G "Pulsetronic" safe lock which detected a series of "knocks" from a keypad. It was nice because the door did not need a spindle hole (but still needed a hole for the boltwork handle). S&G marketed these from about 2000 to maybe 2005 then quietly dropped them. The locks are still made but not marketed aggressively (and not by S&G).

The reason for this post is that a few people have re-invented this in a DIY/kluge manner and posted videos on YouTube. Do a search for "secret knock detecting" (without the quotes).